Some ultrasound transducers utilize a single-layer of piezoelectric material to form the transducer elements. Single-layer transducers have the disadvantage that when operated at higher frequencies, the layer's impedance increases greatly so that a mismatch in impedances occurs between the transducer and the ultrasound system to which it is coupled. Due to this mismatch of impedances, the transfer of energy to the transducer is decreased due to reflection of energy by the transducer.
Ultrasound transducer having multiple layers of piezoelectric material are also known. In some ultrasound transducers the layers of piezoelectric material are uniform in thickness. These transducers with uniform thickness piezoelectric layers suffer from limited bandwidth and poor signal-to-noise ratio due to higher side lobes, especially in in-depth imaging. In addition, they are limited by the lack of control of the slice thickness in the elevation direction.
U.S. Pat. Nos. 5,415,175 ("the '175 patent")and 5,438,998 ("the '998 patent"), both of which are assigned to the present assignee and are specifically incorporated herein by reference, disclose an ultrasound transducer that has two layers of piezoelectric material stacked one on top of the other in the z-range direction as shown in FIGS. 12 and 13 of the '175 and '998 patents. Each layer has a thickness in the z-range direction and a width in the x-elevation direction extending from a first end to a second end. The thickness of each layer is non-uniform, and more particularly, the thickness is at a maximum at the first and second ends and the thickness is at a minimum at a point about midway therebetween. As shown in FIG. 12, the top layer of piezoelectric material has a concave surface which will face the region of examination when the transducer is in use. The bottom layer also has a concave surface which faces a backing block on which the bottom layer is disposed. In the embodiment shown in FIG. 13 the concave surface of the bottom layer faces the top layer of piezoelectric material.
It is thus desirable to provide an ultrasound transducer that has a reduced impedance and an improved electrical match to the ultrasound system to which it is coupled. It is also desirable to provide an interconnect circuit that is simple in construction, maintains the same number of traces as a single layer design and has all of the traces extending from one side of the transducer.